Solar cells including a plurality of relatively small area, spaced apart rectifying junctions are known. For example, a so-called point contact solar cell described by Sinton and Swanson at the IEEE 19th Photovoltaics Specialists Conference (1987) and in the Conference Record thereof at pages 1201-1208 is shown in a schematic perspective view in FIG. 10(a). FIG. 10(b) is a cross-sectional view illustrating the collection of photo-generated charge carriers in the point contact solar cell. FIGS. 10(c) and 10(d) are schematic views showing arrangements of p-type and n-type regions in the point contact solar cell. In these and the other figures, the same reference numbers indicate the same elements.
In FIG. 10(a), a relatively high resistance silicon substrate 8, i.e., a substrate that is relatively free of impurities, receives light, indicated by h.nu., at a front surface and has an array of spaced apart, relatively small area p-type and n-type regions at the opposite, rear surface of the substrate. The n-type regions 1 and the p-type regions 3 are disposed within an insulating film 2 at the rear surface of the substrate. The n-type regions 1 are interconnected through electrodes 6 and the p-type regions 3 are interconnected through electrodes 5. The electrodes 5 and 6 are both disposed at the rear surface. The point contact structure shown exploits the advantages of undoped silicon in which charge carrier recombination rates at the surface and within the bulk material are reduced. The reduced recombination rates produce relatively long diffusion lengths for the photo-generated charge carriers, improving the probability that those photo-generated carriers will reach and be collected at the relatively small area doped regions.
Each of the doped regions 1 and 3 collects charge carriers that reach respective collection regions 11 shown in FIG. 10(b) as spherical regions surrounding respective doped regions. As indicated in FIG. 10(b), these collection regions are not continuous so that charge carriers that travel between the collection regions 11, in "stagnant" regions 12, are not collected. The doped regions are arranged in an array, as illustrated in FIGS. 10(c) and 10(d), to reduce the number and size of the stagnant regions 12 between the collection regions 11. As shown in FIGS. 10(c) and 10(d), the doped regions may be arranged in a rectangular array. A typical distance between adjacent doped regions in a rectangular array is 50 microns.
The open circuit voltage, V.sub.oc, of the point contact solar cell is predicted by the diode equation EQU V.sub.oc =kT/q 1n ](I.sub.L /J.sub.s S.sub.s)+1]
wherein
I.sub.L is the light generated current, PA1 J.sub.S is the dark diode reverse saturation current density, PA1 S.sub.S is the junction area, and PA1 kT/q is Boltzmann's constant multiplied by the temperature and divided by the electronic charge.
In the point contact solar cell, the open circuit voltage is increased, increasing efficiency, because the junction area S.sub.S is smaller than in other solar cell structures.
A method of manufacturing the point contact type solar cell is illustrated in FIGS. 11(a)-11(l). In those figures, a silicon substrate 8 is masked by a first photoresist layer 9 in which apertures are prepared. A dopant is diffused through the apertures into the substrate 8 to form n-type regions 1. After removal of the first mask, a second mask 9 is applied to the substrate and apertures are opened in that second mask at different locations from the n-type regions 1. A dopant producing p-type conductivity is diffused into the substrate through the apertures in the second mask. After removal of the second mask, the structure of FIG. 11(g) is produced.
As illustrated in FIG. 11(h), an electrically insulating film 2, such as an oxide film, is deposited on substrate 8. A third mask 9, for example, a photoresist, is deposited on the oxide film 2 and apertures are formed in that third mask opposite each of the n-type regions 1 and 3. The third mask 9 is employed as an etching mask and the oxide film opposite each of the doped regions is removed by etching through the apertures to provide electrical access to the doped regions. Finally, as illustrated in FIG. 11(l), the electrodes 5 and 6, respectively interconnecting p-type regions 3 and n-type regions 1, are deposited on those regions and on the adjacent portions of the oxide film 2. As shown in these figures, the fabrication of the point contact type semiconductor requires many steps. Photolithographic processing is required three times with increasingly stringent registration constraints. These requirements complicate the fabrication process.